Optical amplifiers for a multiplex signal to be transmitted with channels featuring a number of different wavelengths exhibit a wavelength dependency on gain which is not completely rectified by smoothing filters normally employed. This means that performance differences between individual-channels accumulate as they pass through an optical transmission link. Furthermore the noise figure of these optical amplifiers is also wavelength-dependent. This means that the channels possess greatly differing Optical Signal-to-Noise Ratios (OSNR) at the relevant receivers of the transmission link. In point-to-point connections a method frequently known under the name of “pre-emphasis” for equalizing the values of the signal-to-noise ratios is used, which has been described in A. R. Chraplyvy, J. A. Nagel and R. W. Tkach, “Equalization in Amplifier WDM Lightwave Transmission Systems”, IEEE Photonics Technology Letters, Vol. 4, No. 8, 1992, pp. 920-922. In this method, on the basis of the distribution of the signal-to-noise ratios measured at the end of the transmission link the send-side signal powers of the channels are adjusted in an iterative method until such time as the same signal-to-noise ratios are produced at the end of the transmission link for all channels. An important feature In this case is that the signal powers of all channels can be set at one location.
For long transmission links it can no longer be sufficient under some circumstances to only equalize the signal powers of the channels at the start of the link. Special adjustable filters (DGE=Dynamic Gain Equalizer) have thus been developed which make it possible to equalize the powers in the middle of the link. The same task can also be handled with an add-drop module (OADM=Optical Add/Drop Multiplexer) in which complete demultiplexing on individual channels is undertaken. It is generally assumed that an optimum signal quality at the end of the transmission link is produced if at the end of a link section identical signal-to-noise values are obtained for all channels. Link section here is to be seen as the part of a optical transmission link preferably from the transmitter to the first add-drop module OADM, from an add-drop module OADM to the next add-drop module OADM or from an add-drop module OADM to the demultiplexer with the downstream receivers.
For reasons of cost however it is frequently not sensible to undertake complete demultiplexing down to individual channels; Instead entire bands of channels are looped through by an add-drop module OADM. In this case there is then no opportunity for setting the powers of the channels individually at the location of the OADM, as would be prescribed in a usual method for pre-emphasis.
To provide a better understanding of the problem a simple case is examined below. In the network depicted in FIG. 1 a first e.g. red subband of express channels EK is transmitted from the start Tx of the transmission link LWL to its end Rx, while a second e.g. blue subband of add channels AK—e.g. with smaller wavelengths than in the red subband—is coupled into an add-drop module OADM arranged in the transmission link. The red subband would be looped through in the add-drop module OADM. Regenerators OV1, 0V2 are provided along the transmission link LWL for regeneration of the signals.
Since it is not possible to adjust the power in the add-drop module OADM the initial obvious option is to perform a separate pre-emphasis P1, P2 for the two subbands. And to do this for the blue subband from the add-drop module OADM to the receiver Rx and for the red subband from the transmitter Tx to the receiver Rx. However the problem which arises in this case is that the noise figure of an optical amplifier at long wavelengths (red) can be for example 2 dB greater than that at a short wavelengths (blue). When an average channel power per subband is kept constant the red subband exhibits a far worse performance than the blue subband. This type of method for the individual channel power equalization is described in DE 100 24 393 A1, in which the signal level of express channels, add channels, drop channels or add-drop channels is equalized individually with the aid of a controllable attenuation element at the transmitter or in an add-drop module. In this case a pre-emphasis is initially performed for the express channels. Subsequently the signal level of the at least one optical add-drop signal is equalized with the power spectrum of the express channels determined by means of an intra- or extrapolation. This means that an add-drop signal is inserted in the optimum way as regards the amplitude and the phase into the power spectrum of the express channels determined, however not with respect to performance through noise figure effects.
In FIG. 2 a further problem is looked at preferably for fully-optical networks which also has effects on the method previously described in FIG. 1. express channels EK as red subband are transmitted from a transmitter Tx to a first receiver Rx1 along a transmission link. Two add-drop modules OADM 1, OADM2 are integrated into the transmission link. At the transmitter Tx drop channels DK are injected as blue subband into the transmission link LWL and at the second add-drop module OADM2 dropped from the transmission link LWL and routed to a second receiver Rx2. Regenerators OV are provided for the transmission link LWL.
Both at least one signal of the red subband and also at least one signal of the blue subband are injected at the same location into the network. As part of a pre-emphasis an identical value of the optical signal-to-noise ratio OSNR is forced at the second add-drop module OADM2 for both signals. This does not take account of the fact that signals in the blue subband have covered a much shorter distance after passing the second add-drop module OADM2 than signals in the red subband. express channels EK will thus exhibit a far worse optical signal-to-noise ratio OSNR at the first receiver Rx1 assigned to them than signals in drop channels DK at the second receiver Rx2.
A method is known from EP 0 959 578 A2 for pre-emphasis for a WDM multiplex signal transmitted over a transmission link with add and drop points. At the end of the transmission link a computer will control all pre-emphasis steps centrally, so that absolute signal-to-noise ratios of the signals at receivers connected at the add and drop points of the transmission link are as identical as possible. Relatively different deteriorations of the signal-to-noise ratios e.g. between any given signals transmitted in express channels and in add-drop channels are not taken into account here.